High Hemoglobin and Hematocrit: What Does It Mean and What Comes Next?

High hemoglobin and hematocrit mean the blood contains a larger share of red blood cells, or of the oxygen-carrying protein inside them, than expected. The cause may be temporary, such as dehydration, or long-standing, such as smoking, sleep apnoea or lung disease. Less often, it points to a bone marrow condition called polycythemia vera.

Hemoglobin is the protein that carries oxygen. Hematocrit is the percentage of the blood made up of red cells; a result of 42 means 42% of the blood is red cells. The two usually rise together. The medical terms for a raised red cell level are erythrocytosis or polycythemia.

MedlinePlus notes that the normal range varies with the laboratory, age, sex, smoking status and the altitude where a person lives. Compare your result with the reference range on your own report, and ask the clinician what level they would expect for you. A single abnormal result does not always mean there is a condition needing treatment.

A raised red cell level can make the blood thicker, which increases the risk of blood clots. The NHS and Cleveland Clinic advise getting emergency help for signs of a clot, a heart attack or a stroke:

• Pain, swelling, redness or warmth in one leg.
• Sudden breathlessness, chest or upper back pain, or coughing up blood.
• Dizziness, light-headedness or fainting.
• Sudden weakness, numbness, loss of balance or trouble speaking.

Arrange a prompt appointment if a high result comes with persistent headaches, blurred vision, dizziness, ringing in the ears, red or burning skin in the face, hands or feet, nosebleeds or easy bruising. Itching after a warm bath or shower is a symptom that Cleveland Clinic specifically associates with polycythemia vera. Many people with a high result have no symptoms at all, so the absence of symptoms is not a reason to skip follow-up.

The NHS separates two situations. In apparent (relative) erythrocytosis, the number of red cells is normal but there is less plasma, the fluid part of the blood, so the cells are more concentrated. In absolute erythrocytosis, the body is actually making too many red cells.

Apparent erythrocytosis is linked to dehydration, diuretic medicines (water tablets), smoking, heavy alcohol use and being overweight. Absolute erythrocytosis is either primary, where the problem starts in the bone marrow, or secondary, where something outside the marrow drives extra red cell production.

Yes. Each of these is listed by MedlinePlus, Cleveland Clinic or the NHS as a possible cause or contributor:

• Dehydration: MedlinePlus calls it the most common cause of a high hematocrit. Vomiting, diarrhea and diuretics can contribute.
• Smoking: raises hemoglobin, partly through carbon monoxide, which reduces the oxygen reaching tissues.
• Sleep apnoea: repeated pauses in breathing during sleep can lower oxygen levels and prompt more red cell production.
• Testosterone and anabolic steroids: Cleveland Clinic states that taking testosterone can lead to erythrocytosis, and lists other performance-enhancing drugs too.
• High altitude: thinner air with less oxygen leads the body to make more red cells.
• Lung and heart disease: conditions such as COPD or congenital heart disease that lower oxygen levels.
• Kidney problems: some kidney conditions and, rarely, tumors that release the hormone erythropoietin (EPO).

Polycythemia vera is a rare, slow-growing blood cancer in which the bone marrow makes too many red cells. The NHS notes it is usually caused by a change in the JAK2 gene, is not usually inherited, and is most often diagnosed around the age of 60. MedlinePlus Genetics states that nearly all people with polycythemia vera have a JAK2 mutation, acquired during life rather than passed down in families.

A JAK2 test looks for this gene change in a blood sample. A hematologist may use it to help confirm or rule out polycythemia vera. It is one piece of the assessment and is interpreted with the blood count, EPO level and history, not on its own.

The first step is often a repeat full blood count, ideally when well hydrated, together with a review of smoking, alcohol, medicines and any testosterone or anabolic steroid use. It helps to bring earlier blood results, a full medicine and supplement list, and a note of snoring, daytime sleepiness or breathlessness, since these can point the assessment in a particular direction. Do not stop a prescribed medicine to change the result without discussing it first. Cleveland Clinic lists other tests that may follow:

• Erythropoietin (EPO) level: low levels suggest polycythemia vera, while high levels usually point to a secondary cause.
• JAK2 mutation testing, if polycythemia vera is a possibility.
• Pulse oximetry to check blood oxygen levels.
• Kidney and liver function tests, a urine test and sometimes imaging.
• A bone marrow biopsy in some cases where polycythemia vera is suspected.

Writing these down before the appointment can help:

• How far above this laboratory's range are my hemoglobin and hematocrit, and how do they compare with earlier results?
• Could dehydration, smoking, my medicines or testosterone use explain the result?
• Should I be assessed for sleep apnoea or a lung or heart condition?
• Do I need an EPO level or a JAK2 test, and what would each result change?
• Should I see a hematologist?
• Which symptoms mean I should seek help before the next test?

On SameCase, patients and verified doctors share de-identified cases. Reading how others with a high hemoglobin or hematocrit described their history, such as smoking, sleep problems or testosterone use, and which tests followed, can help you spot details worth raising. Another person's case cannot explain your own result, so bring your questions to the clinician who knows your history.